The present invention relates to an apparatus for detecting a voltage across a thyristor in an alternating-current resistance welding machine.
In an alternating-current resistance welding machine, a power voltage is supplied to a primary coil of a welding transformer by means of a thyristor. From a secondary coil of the welding transformer, a welding current (secondary current) flows through a workpiece to metallurgically join the weld zone of the workplace with Joule heat. The magnitude of the welding current is controlled by changing the firing angle of the thyristor.
To measure the welding current and/or power factor angle of the resistance welding machine, it is necessary to detect when the welding current starts and stops flowing in each cycle. When the thyristor is conductive, the welding current flows whereas when the thyristor is nonconductive, no welding current flows. Thus, the voltage across the thyristor (cathode-to-anode voltage) has an opposite phase to that of the welding current. In this view, the voltage across the thyristor is detected for reading the conducting interval of the welding current.
To this end, the prior art uses a step-down transformer, the primary coil of which is connected across the thyristor. The secondary coil of the step-down transformer is connected to a waveform converter comprising a photo coupler or the like for converting the voltage waveform across the thyristor into a standardized waveform (typically binary or rectangular waveform).
The power voltage is a high voltage having a peak of say 200 .sqroot. 2 or 400.sqroot. 2 volts. The voltage across the thyristor has a similar peak. The secondary voltage across the secondary coil of the step-down transformer has a waveform similar to that across the thyristor. The secondary voltage waveform has a peak of say 20.sqroot. 2 volts. When such secondary voltage is applied to the light emitting diode of the photo coupler, it emits a light, causing the semiconductor output device of the photo coupler (e.g., photo transistor) to become conductive. Thus, the semiconductor output device of the waveform converter generates a rectangular voltage signal having TTL level (0-5 volts). The timings of the rising and falling edges of the rectangular signal indicate the timings of the start and stop of the welding current conduction, respectively.
As stated, the prior art apparatus for detecting a voltage across the thyristor requires a step-down transformer dedicated to step down the high voltage across the thyristor to a reduced voltage appropriate for the waveform converter (photo coupler). This necessarily results in a large-sized and expensive resistance welding control apparatus. Furthermore, an electric current flows through the secondary circuit of the step-down transformer even when no welding operation is performed, thus wasting the electric power.